The present invention relates to the field of optics and in particular to a polarisation beam splitting device whose incorporation into optical systems that require polarisation beam splitters, can significantly reduce their overall dimensions.
Beam splitting devices are commonly used in the field of optics when it is required to separate two spatially overlapping beams of light or two polarised components of a single beam. The prior art teaches of various methods for achieving such a result that employ either blocks of birefringent material, polarisation dependent coatings or other polarisation effects.
When requiring the polarisation components of a light beam to be resolved, the most efficient manner is to employ a block of birefringent material. A birefringent material is one that is optically anisotropic in that the optical properties it exhibits depend upon the polarisation and propagation direction of the incident light. Many crystalline substances, such as rutile, calcite or yttrium orthovanadate, exhibit such birefringent properties and so provide ideal media from which to develop polarisation beam splitting devices. Such crystal structures are so suited for producing compact birefringent medium, as they comprise high-density structures that lend themselves to cutting, so producing incident surfaces and optic axis of the required predetermined orientations. Nicol prisms and Glan-Foucault prisms are examples taught in the prior art of birefringent crystals employed as beam splitters.
FIG. 1 presents a side elevation of a typical block of birefringent material 103 as taught in the prior art. Here an unpolarised incident beam 140 is incident on the block of birefringent material 103, thereby being resolved into two light beams having orthogonal linear polarisations. For reference a propagation axis L is defined corresponding to the axis of an input beam 140. With this particular orientation, beam 140a corresponds to the ordinary beam while beam 140b corresponds to the extraordinary beam. As is typical in optical systems, components are designed such that where possible input and output faces are perpendicular to the central axis L. Therefore, with the incident beam 140 perpendicular to the block of birefringent material 103 the resulting ordinary beam 140a passes without deviation through the block 103 while the extraordinary beam 140b is refracted as shown.
An inherent disadvantage of such a splitting of the ordinary and extraordinary component beams is that when incorporated into an optical system, such blocks of birefringent material 103 introduce an asymmetric beam splitting. It is normally advantageous for the emerging ordinary 140a and extraordinary beams 140b to be parallel and equidistant from the propagation axis L. The dimensions of the other optical elements of an optical system are then directly dependent on the block of birefringent material 103.
By way of example such blocks of birefringent material 103 are considered herein as incorporated with an optical circulator. However, as will be obvious to those skilled in the art, the problem of reducing the dimensions of an optical system that ernploys such a block of birefringent material 103 as a beam splitter, is not limited solely to optical circulators. Such optical systems also include for example, optical isolators and polarisation beam splitters/combiners.
An optical circulator is a device that has at least three ports for accepting optical fibres. Light that enters the circulator through the first port exits through the second port; light that enters through the second port exits through the third. The optical circulator is an inherently non-reciprocal device. If light enters through the first port it exits through the second, but if that light is subsequently reflected back into the second port, it does not retrace its path back to the first port, but exits through the third port instead.
Circulators are necessary, for example, to use the same fibre for both receiving and transmitting data. The first port may be connected to a data transmitter, and the second port to a long distance optical fibre. In that case, data can be sent from the transmitter to the fibre. At the same time, incoming optical data from the long distance fibre enters the circulator through the second port and is directed to the third port where a receiver may be connected.
An optical circulator found in the prior art is that taught by Li et al in U.S. Pat. No. 5,930,039, see FIG. 2, the contents of which are incorporated herein by reference.
This document teaches of an optical circulator 100 that employs reciprocal and non-reciprocal polarisation rotators 130a and 130b, birefringent optical components 103, 108 and 111, and a polarisation dependent refraction element 150 comprising of two tapered birefringent plates 106 and 107. In the preferred embodiment the optical circulator 100 has its optical components aligned such that effects of the birefringent optical components occur in the vertical plane while the effects of the polarisation dependent refraction element occur in the horizontal plane.
The first and third fibres 100a and 100b are inserted in parallel and adjacent to each other into a glass capillary 101 which is followed by a first lens 102. Together the glass capillary 101 and the lens 102 comprise a first collimator 120a. A first block of birefringent material 103, a first compound polarisation rotator 130a, a light guiding device 150, a second birefringent block 108, a second compound polarisation rotator 130b and a third block of birefringent material 111 are then located along a longitudinal axis L of circulator 100. A second collimator 120b comprising a second lens 112 and a second glass capillary 113 which holds the second fibre 114 are found at the opposite end of device 100.
FIG. 3 provides alternative elevations of the optical circulator 100. In particular FIG. 3a presents a side profile of the circulator 100 presenting light propagating in the z-y plane from the first fibre 100a to the second fibre 114. Initially the light propagates through the first lens 102 and into the first birefringent block 103 Walk off within the block 103 in the z-y plane then produces two mutually orthogonal linearly polarised beams, 140a and 140b, as shown. These linearly polarised beams 140a and 140b then propagate through the first compound polarisation rotator 130a before continuing on through the optical circulator 100 until they are recombined by the third birefringent block 111 and focused by second lens 113 into second the fibre 114.
For the optical circulator 100 to work correctly it requires that any light entering the device at the second fibre 114 exits the optical circulator 100 via the third fibre 100b, and not via the first fibre 100a. The non-reciprocal nature of the device lies in the inherent properties of the compound polarisation rotators 130a and 130b. To illustrate these features FIG. 3b presents a side profile in the z-y plane of the circulator 100 presenting light propagating from the second fibre 114 to the third fibre 100b. 
Comparison of the orientations of the linearly polarised electric field components after propagating through the compound polarisation rotators 130a and 130b shows how the polarisation orientation of an electric field depends on which direction it has propagated through the compound polarisation rotators 130a and 130b. The origin of this non-reciprocity lies in the inherent properties of the Faraday rotators 105 and 110. Unlike the half wave plates 104a, 104b,109a and 109b which reverse the rotation experienced by a linearly polarised electric field on reversal of its propagation direction, a Faraday rotator is designed to always rotate a linearly polarised electric field in the same sense irrespective of propagation direction.
FIG. 3c shows the x-y plane profile of light propagating from the first fibre 100a to the second 114, along with that propagating from the second fibre 114 to the third 100b. Initially the light beam from the first fibre exits the first lens 102 at an angle xe2x96xa1 to the x-axis. On exiting the compound rotator 130a each of the linearly polarised beams, 140a and 140b propagate at an angle xe2x96xa1 relative to the x-axis. The angle of propagation of each of these components is then altered by the light guiding device that exhibits extraordinary refractive index ne and an ordinary refractive index no, where no greater than ne.
Tapered plate 106 has an optic axis OA1 that is orientated parallel to the z-axis while tapered plate 107 has an optic axis OA2 parallel to the x-axis. This results in both electric field components of the beams 140a and 140b exiting the light guiding device 150 parallel to the y-axis.
The second block of birefringent material 108 has an optical axis that is also orientated parallel to the z-axis. Therefore, the electric field components of the beams 140a and 140b are both orientated as ordinary rays relative to the birefringent block 108 and so propagate undeviated through it.
The situation is somewhat different in the x-y plane when considering light propagating from the second fibre 114 to the third 100b. The non-reciprocal nature of the compound polarisation rotators 130a and 130b is employed by the light guiding device 150 and the second birefringent block 108 in order to translate light from the second fibre 114 to the third 100b. Before entering the second birefringent block 108 the two electric field components of the light beams are linearly polarised parallel to the z-axis and therefore the beams 140a and 140b act as extraordinary rays within the second birefringent block 108. This results in them being spatially translated along the x-axis before propagating through the light guiding device 150. Translation through the light guiding device 150 imposes an angle xcfx86xe2x96xa1 between the linear polarised beams of light and the y-axis. The light then continues on through the optical components being recombined by the first birefringent block 103 before being focused by the first lens 102 into the third fibre 100b. 
Analysis of FIG. 3 highlights the inherent disadvantage of employing traditional blocks of birefringent material within this optical circulator 100. It is seen that the collimators 120a and 120b are spatially displaced along both the x and z-axes. The result of such an offset in the collimators 120a and 120b is two fold. In the first instance it makes the optical circulator 100 more difficult to align than if the collimators shared a common axis. Secondly, it restricts the minimum dimensions available for the device. Since cost is directly related to the dimensions of an optical component the offset of the collimators adds additional cost to the manufacture of such an optical system.
By redesigning the blocks of birefringent material such that the optical circulator has its collimating elements on a common longitudinal axes the elements of an optical circulator can be made smaller, thus the entire optical circulator is cheaper and easier to manufacture as well as being simpler to align.
In view of the above, it is an object of the present invention to provide a polarisation beam splitting device formed from a block of birefringent material. The beam splitting device resolves a randomly polarised input beam of light into ordinary and extraordinary linearly polarised beam components that propagate symmetrically about an axis as defined by the input beam.
It is a further object of the present invention to provide a compact and economical optical system that employs the aforementioned beam splitting device, such that all the optical elements of the system share a common longitudinal axis.